Publications and other reference materials referred to herein are numerically referenced in the following text and respectively grouped in the appended Bibliography which immediately precedes the claims.
Optical coherence tomography (OCT) is a 3D imaging method that is mainly associated with the production of high resolution 3D images of multilayer semi transparent samples. This method was first demonstrated in the context of ophthalmology [1]. OCT development and application to the biomedical and biological optical imaging fields is rising every day. OCT relies on the light temporal coherence, interference and matter reflectivity (refraction index discontinuity) which are used to measure micro morphology of objects in a turbid environment [2]. Most OCT systems can be categorized into two main groups, time domain (TD) [1] and frequency domain (FD) OCT [3, 4]. The FD-OCT is a more recent technique for 3D imaging. This method has several advantages over its TD counterpart; the most prominent advantages are its high speed and improved signal to noise ratio (SNR) [5-7].
In the time domain OCT (TD-OCT) mode, an incoherent light source is used to illuminate the sample. The observed sample is placed at the focal plane of one of the interferometer objective lens, whereas the second objective is focused onto a reference mirror. In order to obtain the depth morphology of the sample, the reference mirror is scanned along the axis of the objective lens and through its depth of field (DOF). For each microstructure of the sample a strong interference peak is generated when the optical path between the reference mirror and the microstructure is zero. In order to obtain the entire 3D sample structure, the objective lens of the sample is scanned along the lateral dimension of the sample; for each lateral coordinate the depth information is extracted by actuating the reference mirror, as described above. TD-OCT was demonstrated with ultra-high axial resolution [8-9]; however, since the objective lenses are required to have large depth of field, the lateral resolution obtained is usually not very high; a disadvantage which is not apparent in full field OCT (FF-OCT) [10-12].
In the frequency domain OCT (FD-OCT) mode, the sample is either illuminated by a wavelength tunable source which is swept along the wavelength range (SS-OCT) [4] or by a broad band source (Spectral Domain OCT, SD-OCT) [3]. In both cases, the reference mirror is placed exactly at the focal plane while the sample objective lens is focused onto the sample top surface. In the SS-OCT mode, the light source of the system is swept and for each wavelength the interference signal is recorded at different time slots. Then, by applying the inverse Fourier transform (IFT) to the stored data the depth morphology of the sample is revealed. In the SD-OCT modality, the sample is illuminated by a broadband spectral source and the interference signal is recorded using a spectrometer. Then, by applying the IFT operation to the recorded spectra the sample depth morphology is revealed. Both in SS-OCT and in SD-OCT, in order to obtain the 3D sample structure, the sample must be scanned along the lateral dimension; a disadvantage which is not apparent in full field FD-OCT (FD-FF-OCT) [13-16]. In conventional FD and TD OCT systems, the lateral scanning is time consuming and requires that the interferometer (or sample) is completely stable; a fact which might be problematic in many applications.
Among all of the OCT techniques [17], the full field OCT (FF-OCT) technique is the only one that does not requires any lateral scanning. In this method the entire field of view of the sample is imaged onto a parallel detector (CCD/CMOS) and an interference 2D image is recorded simultaneously; which allows ultra-high speed enface OCT imaging [16, 18]. In the full field TD-OCT (TD-FF-OCT) mode, the observed sample is placed exactly at the focal plane of one of the objective lenses. The reference objective is focused onto the reference mirror. Then, by actuating the reference mirror just about the focal plane and over 2π radians of the phase difference, usually four phase shifted images are grabbed in four different time slots [19-20]. By a simple combination of these phase shifted images, the OCT en-face images are obtained at high speed (video rate). In order to obtain the entire 3D image, the sample is moved one step toward the sample objective lens and the above process repeats itself. By repeating this process for as many depth sections as are needed, the entire sample 3D structure is revealed. The TD-FF-OCT usually uses high numerical aperture objectives as it does not require large depth of field for the imaging. This fact, including the fact that no lateral scanning is involved, instills the method microscopic properties and therefore it is often referred to as optical coherence microscopy (OCM), instead of OCT. The high lateral resolution and the avoidance of lateral scanning are beneficial in some applications. However, as the phase shifted interference images are acquired in different time slots, the interferometer microscope must be kept completely stable; a fact which might be very problematic for some applications. Also, as each en-face OCT image requires four (at least three) phase shifted images, which are grabbed in different times, the imaging speed is not brought to its full potential. Several studies were published on TD-FF-OCT using few parallel detectors [21-22].
In a similar manner to the conventional TD-OCT, the FF-OCT has also an equivalent in the frequency domain mode; the full field Fourier domain OCT (FD-FF-OCT) [13]. In the FD-FF-OCT mode, the illumination is provided by a wavelength tunable light source; either a tunable filter or tunable laser. The observed sample is placed at the focal plane of one of the sample's objective lens of the interferometer such that its top surface is coincident with the focal plane of the objective. The reference mirror is positioned right at the focal plane of the second objective lens of the interferometer. The sample and reference reflected fields are interfered and an interference image is projected onto a parallel detector. For each wavelength an interference image is recorded. Then, by applying the IFT operation to each pixel the 3D morphology of the structure is obtained. The FD-FF-OCT is the only method among the OCT methods which does not require any form of scanning, neither axial nor lateral. As such, this method has very low mechanical noise and can be designed to have ultra-high 3D imaging speed by choosing high speed cameras together with fast swept tunable filter or tunable light source [16].
However, despite the fact that FD-OCT has very high speed and higher SNR when compared to TD-OCT, it also suffers from some drawbacks. The FD-FF-OCT is not exceptional in this manner and like all FD-OCT techniques it also suffers from the following drawbacks: (1) As the FD interference signal is basically a train of raised cosines (real and even function), the IFT operation results in a completely symmetrical image by which each sample structure is identified by two equivalent impulses. The impulses are located in symmetry on both sides of the zero delay line (focal plane). Therefore, the obtained depth images are obscured and cannot be easily interpreted. (2) Due to the high coherence length of each spectral line, the IFT images contain a parasitic noise which originates from the sample internal reflections. (3) To avoid mirror images obscuration, only half of the depth of field of the objective is used for imaging, typically in these cases the imaging range is not brought to its full potential. In the past, several researchers have tackled these problems in different ways. The first work to approach this problem used a SD-OCT system and a piezoelectric transducer at the reference arm by which five phase shifted signals were grabbed in different time slots [23]. Then, by simple combination of these signals, the full range FD-OCT images were obtained without significant mirror images, as well as reduced DC and coherent noise. This method was also demonstrated in-vivo on the human eye [24], and later was improved by using only two phase shifted signals [25]. In the following years researchers consistently tried to develop better techniques for obtaining clear FD-OCT images with full range imaging. These included methods using acousto-optic modulator (AOM) [26-27], tilted reference mirror modulation [28], 3×3 fiber coupler which delivers 3 phase shifted interference signals [29], polarization demodulation using a fiber Mach-Zehnder interferometer [30], lock-in technique with a single detector and a monochromator [31], a method called BM mode [32] which required the mechanical modulation of the reference mirror, a lateral Hilbert transform [33], demodulation through lateral scanning together with lateral Hilbert [34] and Fourier [35] transforms, piezoelectric fiber stretch demodulation [36], an iterative dispersive encoded full range algorithm [37] and also a method using an optical delay line [38]. However none of the above methods is suitable for the full field OCT configuration and, to the best knowledge of the inventors, was not employed in the past by any means.
It is a purpose of the present invention to provide an OCT method and system in which the phase shifted images are grabbed simultaneously and therefore does not require a highly quiet environment.
It is another purpose of the present invention to provide an OCT method and system that provides at least a three-fold faster or more imaging speed than previously proposed methods and systems.
It is another purpose of the present invention to provide an OCT method and system in which the optical setups, as well as the algorithms, are completely different from those disclosed in previously proposed methods and systems.
It is another purpose of the present invention to provide an OCT method and system that overcomes the problems that have previously arisen using the FF-OCT imaging modality.
Further purposes and advantages of this invention will appear as the description proceeds.